The present invention relates generally to apparatus for the reception of carrier signals and, more specifically, to apparatus for sampling a plurality of carrier signals at diverse times in order to overcome the potential ambiguity of valid carrier signals during periods of short duration electrical noise which is common to more than one phase of a polyphase electrical transmission system.
The use of high frequency carrier signals to transmit data along transmission lines is known to those skilled in the art. In systems of this type, a high frequency carrier signal, such as a 12.5 kilohertz carrier signal, is imposed on a 60 hertz electrical current and transmitted along a power line. At a location which is generally remote from the transmitter of the carrier signal, a receiver removes the carrier signal from the 60 hertz current, by passing it through a high pass filter, and then examines the carrier signal in order to extract digital data. Various types of carrier signal demodulators can be used to extract the digital data from the carrier signal. One such demodulator is disclosed in U.S. Pat. No. 4,311,964 which issued to Boykin on Jan. 19, 1982. U.S. Pat. No. 4,311,964 discloses an apparatus and method for coherent phase demodulation of a binary phase shift keyed carrier. By sampling the incoming signal at a known sample frequency, it is possible to mathematically deduce the existence of a carrier signal and to extract its digital message from a background which may include significant electrical noise. Improved demodulators, which are appropriate for examining carrier signals, are disclosed in U.S. Pat. Nos. 4,514,697 and 4,516,079 which were filed on Feb. 9, 1983 by York and issued on Apr. 30, 1985 and May 7, 1985, respectively. U.S. Pat. No. 4,311,964 and U.S. Pat. Nos. 4,514,697 and 4,516,079 are hereby incorporated by reference.
The electrical noise normally found on power lines contains components which are periodic and aperiodic, impulsive and continuous. Periodic noise of both the impulsive and continuous types usually has a period which is some harmonic of the power frequency, such as 60 hertz, and originates from loads on the power line such as SCR'S, rotating equipment, etc. Aperiodic noise is generally impulsive and is the result of random events such as switch closures and electrical lightning. Impulsive noise is normally dominant on power lines. Most power line carrier signals differ from noise in that they are usually continuous waveforms such as those employed by frequency shift keyed or phased shift keyed modulation and they avoid high energy content at harmonics of the power line frequency. The sampling frequencies utilized by the aforementioned demodulation patents are specifically selected to minimize the impact of most impulsive noise by separating the samples by multiple carrier cycles.
In some applications of carrier signal communications, a signal is sent from a transmitter which is operatively connected to a single phase electrical system, such as that of a personal residence, and received by a receiver which is located at a remote location and operatively connected to a three phase power line system. In addition, a carrier signal transmitted on one phase of a polyphase electrical transmission system can be coupled to one or both of the other two phases. The strength of these coupled signals are dependent on many factors, such as the length of transmission line over which the signal is present, the configuration of that power line, the frequency content of the signal along with other power line characteristics. These coupling mechanisms generally act equally on carrier signals and electrical noise. Under these circumstances, the receiver can receive and monitor signals on any one or all three of the phases available to it. For this reason, it is preferable for a receiver to be configured in such a way that it is capable of monitoring signals on all three phases of the electrical transmission line. U.S. Pat. No. 4,382,248 which issued to Pai on May 3, 1983 discloses such a polyphase receiver and is hereby incorporated by reference.
The aforementioned coupling of carrier signals between associated phases of a polyphase power line system can be used advantageously by a receiver. The receiver can be connected in such a way that it can monitor signals on all of the phases and these signals can be combined in any one of a number of known techniques in order to result in a single carrier signal which can be decoded to determine its digital message.
A significant benefit can be achieved by having a receiver monitor all phases of a polyphase power line system. The benefits of this type of polyphase reception system can be best realized by comparing it to an alternate system which only monitors one phase. In the single phase receiver system, the presence of electrical noise on the monitored phase can cause a demodulator to miss a valid carrier signal. Although sophisticated techniques for interpreting carrier signals have been developed and are disclosed in the above-mentioned U.S. Pat. No. 4,311,964 and U.S. Pat. Nos. 4,514,697 and 4,516,079, the possibility remains that electrical noise impulses on the transmission line can exacerbate the demodulation of the carrier signal or extend the length of time required in order for a demodulator to determine that a valid carrier signal is present. By comparison, when the receiver is monitoring carrier signals on all phases of a three phase power line transmission system, the carrier signals received on each of the three phases can be compared and combined in order to enhance the determination of the presence or absence of a signal at any given instance of time. If electrical noise is present on one of the three phases, a combination of that signal with the signals received on the other two phases can be made to diminish the effects of that noise on the demodulation process. Therefore, the concurrent monitoring of all phases by a receiver is advantageous when spurious electrical noise exists which is different in content on each of the three phases.
Even in receiver systems which monitor all phases of a polyphase transmission system, electrical noise can make the demodulation and interpretation of the carrier signal more difficult by appearing as erroneous samples within the carrier signal. This problem occurs, even when the receiver is monitoring all three phases, when similar electrical noise samples appear simultaneously on all phases of a polyphase electrical transmission system. The simultaneous appearance of electrical noise pulses on all three phases can occur in at least two ways. First, if all three phases of a power line are disposed proximate a source of electrical noise, that electrical noise can affect all three phases in a similar manner. A second, more common, cause for electrical noise appearing coincidentally on all phases of a power line is that, when electrical noise exists on one phase of a three phase system, that noise is coupled to the other phases in a manner similar to that which couples carrier signals between the phases of a three phase system. The degree of coupling between phases is dependent on the same multiplicity of factors that applies to signal coupling.
Therefore, even though receivers which monitor all three phases of a three phase power line system offer significant improvement over single phase receivers, there are conditions and situations in which even three phase receivers can be adversely affected by electrical noise which, to the demodulator of the receiver, could mask the demodulation of a valid carrier. This problem occurs in three phase receivers when the electrical noise is coincident to all three phases at any specific sample time. If the electrical noise appears in samples from all three phases at any specific time, the noise is given a much greater weight in the demodulation process than if it appeared on a distributed basis. A significant improvement could be realized if a three phase receiver could reduce the probability of a signal noise impulse, common to multiple phases, from generating erroneous samples on all of those phases.
The present invention makes it possible to sample all phases of a three phase system in a manner which minimizes the probability that multiple erroneous samples will occur due to a single noise impulse existing simultaneously sampling on a time diversity basis increases the probability of at least one phase of a three phase receiver having a signal-to-noise ratio much greater than the remaining two phases. This is accomplished by providing a preselected time delay between the sampling of the phases. While providing time diversity between samples on the different phases, the present invention maintains a consistency of timing between sequential samples taken on any specific phase.
In order to provide a time delay between the sampling of each of the three phases, while also maintaining a consistent sampling frequency on each of the phases, the present invention utilizes a device, such as a shift register, which is capable of shifting a string of digital data. The use of a shift register enables the present invention to maintain a pattern of bits which is sequentially shifted as a function of time. As the data is shifted through the shift register, preselected outputs of the register are used to activate sampling mechanisms which are cooperatively associated with each of the phases.
A periodic sample pulse is used as a data input to the shift register in order to change the logic level of the data being shifted through the register. A periodic clock pulse is used to cause the actual shifting of the data. Of course, the clock pulse frequency must be higher than the sample pulse frequency. In the present invention, the clock pulse frequency is chosen to be an integer multiple of the sample pulse frequency and each occurrence of a sample pulse is coincident with an occurrence of a clock pulse to provide synchronization between the clock pulse and sample pulse frequencies.
As the sample pulse proceeds through the data string of the shift register, different outputs of the shift register are affected by any given data bit at different times. Therefore, as the sample pulse proceeds to the first preselected output of the shift register, its logic level will be transmitted to the first phase sampling means. Sequentially, as that same sample pulse is shifted through the shift register, it eventually reaches a second output and the logic level of the sample pulse will activate a second phase sampling means in much the same way as it previously affected the first phase sampling means. By determining the frequency of both the sample and clock pulses, the frequency of sampling for each phase can be determined. Furthermore, the choice of outputs of the shift register will determine the time delay between the sampling of the phases.
By sampling the three phases on a time diversity basis, electrical noise pulses which occur simultaneously on the three phases will not appear in all three phase samples. In this way, the data can be demodulated and interrogated without a single short burst of electrical noise appearing as multiple sample errors to the demodulator.